Tachometer



C. G. DAVEY TACHOMETER March 1, 1932.

Flled July 14, 1928 Patented Mar. 1, 1932 uNirED STATES CLARENCE GEORGE DAVEY, F FLINT, MICHIGAN, ASSIGNOR TO C SPARK PLUG COItT- I IPANY, OF FLINT,[1\IICHIGAN, A. COMPANY OF MICHIGAN v TACHODIETER Application filed July- 14,

This invention relates to tachometersused to give visible indication of the speed of rotating members with which they are associated.

The object of this invention is to increase the magnetic field of such an instrument and thereby to make it more sensitive and its readings at low speed more accurate. Other objects, such as simplicity in arrangement and economy in manufacture, will appear from the following description.

In the drawings: v

Figure 1 is a sectional view diametrically thru the improved tachometer; and

Figure 2 is a fragmentary sectional view substantially on line 22 of Figure 1.

Referring to the drawings by reference characters, numeral 12 is a cup-shaped frame having an apertured stem 11 for the passage of a rotor shaft 10. The rotor shaft is apertured to receive a driving connection with a' suitable shaft, preferably. a flexible shaft, which latter is to be connected to the rotatable member, the rate of rotation of which is to be measured.

The rotor shaft may have an annular groove 3 to receive a key 5 carried by the stem 11 to prevent axial movement of said shaft 10. The stem 11 may be provided with a recess 21 containing a suitable wick 23 held in position by a cup 25. The purpose of the wick is to carry lubricant for lubricating the engaging surfaces of the stemll and shaft 10. An outer casing 27 houses the frame 12 and its sleeve 29 is sufficiently extended to retain both the key 5 and the cup 25, as shown in Figure 1. This casing 27 is secured to the frame by a plurality of'bolts 31 and nuts 33. The bolts pass thru lugs or projections 35 and 37 formed on the frame 12, as shown in Figure 2. There will be athird projection diametrically opposite lug 37 Spacing means 39 on the bolts between the casing27 and the frame 12, together with a washer 41 and an annular fabric washer 43 between the casing and the frame, serve to secure these parts together in" dust-tight relation.

An annular shoulder 45 in the frame supports a U-shaped magnet 13, the magnet hav- 1929. Serial No. 292,792.

ing its inter-polar gap adjacent bolt 31 in the projection 35. A temperature compensat ing plate ofknown characteristics isplaced over the magnet poles. To hold the magnet V in positon on the shoulder 45 and to also hold the compensator on the poles of the magnet, a clip 46 is formed with an aperture by which it is dropped over bolt 31 and upon the extension 35. The washer 49 and nut51 clamp the clip in position, its inner end -firm-. ly engaging the compensator plate 46. Similar clips 46 on bolts 31 carried by lug 37'a'nd by the diagonally opposite lug, not shown, are used. These last-named clipsengage the flattened ends 53 of a bridge member 55, which is split as shown in Figure 2. The

ends 53 of this bridge rest on the magnet and the clips hold the parts in assembled relation. r

A rotor 9 is secured in any convenientway to the upper end of shaft 10. As illustrated,

the end of shaft 10 is reduced to receive on the shoulder so formed the opening within the rotor member 9 and thereafter the extreme end of the rotor shaft is 'crimped to hold the rotor in position. The rotor is formed with a plurality of radial arms 57 as shown. WVithin the open end of the rotor shaft is a carrier 59 having in its lower end an end thrust bearing 61 and near its upper; end, a hole jewel 63. Secured by threads in the intermediate part of the bridge is a second bearing carrier having a passage therethrough and a hole jewel 65; The-open ings in the hole jewels 65 and 63 are in alignment and together with end bearingol provide the necessary bearing support for a spindle 67. Secured 'by fastening means 69 tothe spindle 67 is a metallic disc 14 designed to serve as a drag member and to be rotated by'the eddy currents against the tension of a spring 71 secured to said spindle and to a rotatable adjustment. As is wellknown, in such instrumentsthe'degree of rotation of drag disc 14 depends upon the rate at which the rotor 9 travels, cutting as it travels the magneticlines of force between the magnet poles.

A clip 7 3 rests on the bridge 55 and is apertured' to rotate about the extended end 1 of the carrier 65. The clip extends across the bridge and on one side is bent down, as at 75, and secured to the lower end of the bent down portion 7 5 is the outer end of spring 71. The other end of the clip is given a double bend and resiliently engages by its extended end (forked as at 77) the under side of the bridge. Rotation of the clip serves to Vary the tension of spring 71.

At the upper end of the spindle is mounted a part 7 9 carrying an indicating needle 16 balanced by an opposite end part 81. The needle moves over a dial plate 19, the peripheral portion of which engages the marginal flanges of the casing 27. In practice, there Will be a glass cover and a fastening bezel, which parts are conventional and not illustrated. V

In instruments of this type, there is a tendency to inaccuracy at low speeds. It is proposed to correct this inaccuracy at low speeds by increasing the magnetic field. To this end, a second magnet 18 is secured to the base of the frame by fastening means 83. This second magnet thus lies under the rotor 9 and the disc let, while magnet 13 lies above these parts. It will be understood that like poles of the two magnets are located the one opposite the other, a north pole of one magnet being above the north pole of the other. The use of this second magnet increases the intensity of the magnetic field and it is found to aid in giving accurate readings at low rotor speeds. The invention is obviously simple and easy to construct and is quite inexpensive. The magnets, the rotor and the drag disc are all in parallelism wherby an exceedingly compact arrangement results.

I claim:

1. In a tachometer, cup-shaped frame having spaced parallel surfaces, C-shaped magnets mounted in parallel spaced relation on said surfaces, said magnets having their corresponding poles in axial alignment, a rotor shaft extending through the base of said cupshaped frame, a rotor terminally carried by said rotor shaft and positioned in the space between the said magnets, a drag disc also positioned between said magnets, a spindle carrying said drag disc and journaled coaxially with said rotor shaft, said spindle ex tending axially beyond the region of the magnets, and an indicating member carried bysaid spindle.

2. The invention defined by claim 1, one of said parallel surfaces being the inner face of the bottom of the cup-shaped frame and the other surface being an annular shoulder in the side wall of the frame.

3. The invention defined by claim 1, together with independent means to secure said magnets upon their respective surfaces.

In testimony whereof I affix my signature.

G. G. DAVEY. 

